This invention relates to an optical glass having optical constants of a refractive index (nd) within the range from 1.48 to less than 1.55 and an Abbe number (νd) within the range from 45 to 55, having a positive anomalous dispersion of Δθg,F of +0.0010 or over and being particularly suitable for reheat pressing.
A lens system of an optical instrument is normally designed by combining lenses having different optical properties. A lens consisting of glass having a positive or negative anomalous dispersion is used for correcting secondary spectrum and a glass of a low refractive index having a positive anomalous dispersion is technically important because it can extend freedom of design for lens systems of optical instruments the type of which is of an increasing variation, though the quantity of production of this type of glass is not so large at present.
For producing an optical element such as a lens from an optical glass, which is not manufactured in a large scale, a general method is as follow which economically suited for a small scale production of many types of products.    Process 1: At room temperature, cutting or splitting or grinding an optical glass which has been formed to a plate or block to a glass gob of a predetermined weight.    Process 2: Reheating the glass gob (obtained by process 1) to soften and pressing the softened glass gob to a shape resembling the shape of a desired optical element    Process 3 Grinding and polishing the press mold product (obtained by process 2) to the shape of desired optical element and applying coating on its polished surface.
Temperature of glass which corresponds to viscosity of the glass suitable for the reheat pressing is a temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 300° C. The reheat pressing is conducted by placing glass gob having a temperature which is held within the above described temperature range in a press mold which is held at a temperature lower than the temperature of the glass for preventing fusing together of the glass and the press mold, and press molding the glass. For reducing deterioration of the press mold caused by contact of the glass of a high temperature with the press mold during reheat pressing, a lower glass temperature is preferable. For this purpose, it is preferable to conduct the reheat pressing at a temperature within a temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 250° C. If devitrification or opaqueness caused by phase separation does not take place when the glass has been held for 30 minutes at a temperature within a temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 300° C., the glass can be subjected to the reheat pressing. If devitrification or opaqueness caused by phase separation does not take place when the glass has been held for 30 minutes in a temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 250° C., the reheat pressing is available for that glass with reduced deterioration of the press mold.
In a case of a glass which tends to cause devitrification or opaqueness, when the glass is reheated and held for 30 minutes at a temperature within the temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 300° C., devitrification or opaqueness caused by phase separation takes place. If the glass which tends to cause devitrification or opaqueness is not held at a temperature within the above described temperature range but the temperature at which the glass is held is further raised, occurrence of devitrification or opaqueness is decreased but, when the glass is subjected to the reheat pressing while it is held at a temperature of Tg+250° C. or over, the press mold tends to deteriorate for the reason stated above with resulting reduction in the life of the press mold and, therefore, such reheat pressing is not desirable. When the glass is subjected to the reheat pressing while it is held at a temperature of Tg+300° C., viscosity of the glass is reduced with resulting difficulty in reheat pressing.
Among ingredients of an optical glass, PbO and As2O3 have recently been considered as materials causing adverse influence to the environment. Since waste and sludge of these ingredients which are produced in manufacturing and processing of the glass pollute the environment, special care must be taken for protecting the environment when such waste and sludge are treated as industrial waste. In Europe, it is scheduled to prohibit use of an optical glass containing PbO in future. For these reasons, there is an increasing demand for an optical glass which does not contain these ingredients. Since, however, PbO has a great influence on optical and chemical properties of an optical glass, it is fairly difficult to develop an optical glass which does not contain PbO and yet has optical and chemical properties which are substantially equivalent to those of the prior art optical glass containing PbO.
Glasses of various compositions having optical constants of a refractive index (nd) within the range from 1.48 to less than 1.55 and an Abbe number (νd) within the range from 45 to 55 are disclosed by German Patent No. 973350 which has very broad composition ranges in its scope of claim. Among these glasses of various compositions, there is a SiO2—B2O3—Al2O3—K2O—PbO—As2O3—TiO2—F system glass. Since, however, this glass contains PbO and As2O3, it has the disadvantages that it has the problem of environmental pollution and that opaqueness caused by phase separation takes place when the glass is reheated and held at a temperature within the temperature range which is higher than the glass transition temperature (Tg) by 200° C. to 250° C.
The German patent also discloses glasses which has the above described optical constants and is free of PbO but these glasses mostly contain As2O3 in an amount of 0.3-5.0 weight % which causes the environmental pollution and, further, has at least one of the following disadvantages (1) to (5):                (1) When the glass is reheated and is held at a temperature within a temperature range which is higher than the glass transition temperature by 200° C. to 250° C., opaqueness caused by phase separation tends to take place. (2) Chemical durability of the glass is poor. (3) Light transmittance on the short wavelength side in the visible region is poor. (4) Since the melting property of the glass is poor and melting of the glass at a high temperature is required, fluorine in the glass evaporates with the result that a homogeneous optical glass having desired optical properties cannot be obtained. (5) Since the glass contains a relatively large amount of Sb2O3 ingredient (3.0-15.0 weight %), Sb metal and/or Sb ion tends to alloy with platinum constituting the melting equipment.        
Further, the German patent also discloses optical glasses which have the above described optical constants and is free of PbO and As2O3 but these optical glasses all contain a very large amount of Sb2O3 (20.0-30.0 weight %) and, therefore, these glasses have a stronger tendency to the above described alloying. For continuously producing a high quality optical glass, most optical glasses presently are manufactured by using a melting equipment which is made of platinum at least in a portion which comes into contact with melted glass. If the Sb metal and/or Sb ion alloys with platinum, heat resisting property of the alloyed portion is substantially deteriorated and, as a result, there occurs a danger that a hole is formed in the alloyed portion to let the melted glass flow out of the melting equipment. In that event, the melting operation must be stopped instantly and the glass under melting must be scrapped and, moreover, the melting equipment must be disjointed and the portion where the hole has been formed must be repaired. This repair work stops glass production for a long period of time. Furthermore, since platinum is an expensive material, the alloyed platinum must be refined again and reused. Thus, there occur substantial danger and economic loss caused by the alloying accident.
Japanese Patent Application Laid-open Publication No. 6-92675 discloses specific compositions of an optical glass having optical constants of a refractive index (nd) within a range from 1.48 to less than 1.55 and an Abbe number (νd) within a range from 45 to 55, being free of PbO and As2O3 and containing fluorine. This optical glass however is not free from at least one of the above described disadvantages (1) and (2).
Japanese Patent Application Laid-open Publication No. 8-290936 discloses specific compositions of an optical glass having optical constants of a refractive index (nd) within a range from 1.48 to less than 1.55 and an Abbe number (νd) within a range from 45 to 55 and being free of PbO and AS2O3. This optical glass however, has no positive anomalous dispersion. Moreover, this optical glass has the problem that the glass tends to be colored.
It is, therefore, an object of the present invention to provide an optical glass which has eliminated the above described disadvantages of the prior art optical glasses comprehensively, namely an optical glass having a positive anomalous dispersion which is free of PbO and As2O3 which cause environmental pollution, has optical constants of a refractive index (nd) within a range from 1.48 to less than 1.55 and an Abbe number (νd) within a range from 45 to 55, is free of opaqueness when the glass is reheated and held at a temperature within a temperature range which is higher than the glass transition temperature by 200° C. to 300° C., particularly by 200° C. to 250° C., and can be produced safely without being alloyed with platinum in the melting process.